Joslin "Jos" Reyes grew up in one of the roughest neighborhoods of Queens, New York. She is straight talking and no-nonsense, but has a warm side that she does not share easily. She is also fiercely loyal and someone you really would not want to get on the wrong side of. Upon coming of age, Reyes joined the New York Police Department. However, after being accused for the murder of a local gang leader, Reyes was suspended from the NYPD. Due to a natural affinity with engines, she re-trained and found work as a mechanic.

Reyes has known Conrad Roth for 15 years, and, on occasion, she worked on his ship as chief mechanic, and she has a 14-year-old daughter, Alisha. Alisha often lives with Reyes’ sister when she is off working with her "Uncle Roth", unbeknownst to her that Roth is actually her father. Reyes hates being away from her, but Roth pays better than any garage she has ever worked for. Getting back home to her daughter is always Reyes’ main motivation. One day she would like to make enough money to open her own garage and spend more time with her daughter. Because of Alisha, Reyes will not sacrifice herself for anyone, but will still put herself in danger for people when her academy training kicks in.

Reyes is first seen on the island helping the other survivors from the shipwreck, but does not hear Lara's calls for help due to the storm and the high amount of panic amongst the others.

Amongst all of the survivors of the shipwreck, Reyes is the only one who seems to harbor a grudge against Lara; this is because she blames Lara for the shipwreck, as she thinks that the whole expedition was nothing more than a pipe dream. She even goes as far as to admit this to Lara in front of everyone while mourning Roth after his death (though she later regrets making this comment). After Alex dies aboard the remainder of the Endurance, Reyes again brings up that people around Lara tend to die. However, by the end of the game, Reyes seems to respect Lara for everything she has done to help her and the other survivors.

In her letters and a video recorded by Samantha Nishimura, it shows that she and Roth are involved in a relationship that they hide from the crew. The result of the relationship is Reyes' daughter Alisha, although Roth does not know that he is the father.